The invention relates to a coaxial resonator structure and a filter structure consisting of coaxial resonators.
Air-insulated coaxial resonators are popular e.g. in radio-frequency filters at base stations of mobile communication networks because of their relatively good electrical characteristics and good manufacturability. A coaxial resonator is most often a quarter-wave resonator where one end of the resonator is short-circuited, i.e. the inner conductor and outer conductor of the resonator are in galvanic contact, and the other end is open. When manufacturing a filter comprising a plurality of resonators, it has to be taken into account that the desired frequency response of the filter may require inner conductors of different lengths. In addition, it may be necessary to compensate for the change in the resonance frequency caused by thermal expansion in the resonator structure.
There are several known methods for manufacturing coaxial resonators functioning as filter elements. FIGS. 1 and 1a depicts a known structure viewed from the side and from the bottom. Shown in the figures is the resonator""s outer conductor 111, inner conductor 120, and the bottom 112. The outer conductor and the bottom are shown in longitudinal section. They constitute a single body 110 having a quadrangular cross section and a cylinder-shaped cavity. The body 110 is made by the process of milling or extruding, for example. The inner conductor is in the said cavity, attached to the bottom 112 of the resonator so that the bottom provides the short circuit between the inner and outer conductor. In this example, the attachment of the inner conductor is realized using a screw 130 extending through the bottom to the inner conductor. The inner conductor may be of the same or different material than the body 110. In the latter case, the temperature compensation in the resonator may be realized through the choice of materials. FIGS. 1 and 1a shows only one resonator in a simplified form. A whole filter may have several cavities in one and the same body with electromagnetic couplings between the cavities. On top of the whole structure, there is a conductive cover.
A disadvantage of the structure depicted in FIGS. 1 and 1a are the attachment of the inner conductor takes several manufacturing stages and the attachment strength does not meet the strictest requirements. Moreover, the interfaces between the different metals in the structure may cause harmful intermodulation products when using the device.
FIG. 2 shows a longitudinal section of another known structure. The figure shows the resonator""s outer conductor 211 and bottom 212. The resonator""s inner conductor has now two parts: A lower part 213 nearer the bottom of the resonator and an upper part 220 forming a continuation of the lower part. In this example, the lower part 213 of the inner conductor, bottom 212 and the outer conductor 211 constitute a single body 210. At the bottom end of the upper part of the inner conductor there is a cylindrical hollow 221 the diameter of which equals that of the lower part of the inner conductor. Thus the upper part 220 may be pushed onto the lower part 213 as a cap. The purpose of the upper part of the inner conductor is to provide for the temperature compensation in the resonator. A disadvantage of the structure is that the attachment of the inner conductor requires a separate manufacturing stage. Moreover, in this case, too, the risk of harmful intermodulation is apparent at the attachment interface.
An object of the invention is to reduce the above-mentioned disadvantages associated with the prior art. The method according to the invention is characterized by that which is specified in the independent claim 1. The structure according to the invention is characterized by that which is specified in the independent claims 2 and 9. Some preferred embodiments of the invention are specified in the other claims.
The basic idea of the invention is as follows: a resonator body is manufactured by means of extrusion. In the pushing part of the extrusion machine a recess is made which is shaped like a portion of the surface of the additional object to be attached to the body. The additional object, such as a portion of an inner conductor, is inserted prior to the extrusion in the said recess so that the said portion of the surface of the additional object is positioned tightly against the surface of the recess and the rest of the surface of the additional object remains free. In the extrusion stage, the pushing part with the additional object is pressed against the bloom of body whereby the material of the bloom is pressed tight against the free surface of the additional object. When the pushing part is retracted the additional object remains attached to the body thus formed.
An advantage of the invention is that in conjunction with the manufacturing of the body part of the structure being manufactured, parts may be added to the body without separate manufacturing steps. This means savings in the manufacturing costs. Another advantage of the invention is that the bond according to the invention between the additional part and body part is stronger and more uniform than similar prior-art bonds. This means higher reliability of the structure and less intermodulation at the attachment interface. A further advantage of the invention is that in a filter manufactured according to the invention the inner conductors of the different resonators may be manufactured in different lengths, if necessary, by using recesses of different lengths in the pushing parts of the extrusion machine. The additional parts to be attached to the inner conductors may then be identical, which is advantageous from the standpoint of manufacture.